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/sci/ - Science & Math

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5340954 No.5340954 [Reply] [Original]

Hi, /sci/!
I am not particularly versed in physics, but my unscientific mind's WTF detector went off when I read books on Special Relativity. Could you help me understand this better? I have several questions.

1. Tying space-time to speed of light. SR assumes (amirite?) that before we observe the event, it doesn't happen. I don't understand why it was so absolutely necessary to not regard time as an absolute value. One second passes on Earth = One second passes in a different planet elsewhere = one second passes near inside a black hole's sphere of influence. Does information pass with the speed of light? Yes, but it doesn't mean that the flow of time is related to spread of information.
2. The book I read the last made a big point of an impossibility of calculating absolute velocity of a celestial body. Buuuuut don't we have speed of light as that absolute constant? Why couldn't we calculate our absolute velocity by sending radio signals to six different relays, positioned at the same distance from transmitter and recording time when these signals arrive?
3. Time dilation. The most WTF concept for me.

4. Impossibility of traveling faster than light being based on it violating causation laws. I imagine the situation as:
Event A occurs at point B. Observer is based in point C, which is 1 light year away from point B. Dude D is on point B, witnessing A and going to tell Observer how fukken cool it was. D has a drive that allows him to travel two times faster than light.

So. Event A occurs. Half a year later D arrives to C, telling of an event. Another half a year later D and Observer can see the event occurring while being on C. Where is the problem? Even if FTL was instantaneous, how could it ruin cause and effect link, or help travel in the past?

Well, all of it boils down to "why the hell time is not an absolute value in SR?".

>> No.5340978
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5340978

>>5340954
>>5340954
>first assumtion/question is wrong

>SR assumes (amirite?) that before we observe the event, it doesn't happen

NOPE.
I ain't even gonna read the rest of your bullshit.
Try again.

Think a little harder before you ask shit.

>> No.5340982

to understand SR you have to *start* from the fact that the speed of light is the same in all reference frames. This makes no sense in Newtonian mechanics, but observational evidence for it was piling up in the late 1800s, and special relativity is the simplest way to explain that. Einstein's insight was that time doesn't have to be absolute any more than space is (and we've been accustomed to non-absolute space for a millennium or so).

1. I don't think SR assumes that. It does divide spacetime (from your perspective) into your past light-cone (events which you theoretically could observe, ie which could affect you); your future light cone (events you could affect / which could observe you), and everything else, which is neither in your future nor your past. You can arbitrarily extend your notion of simultaneity out of your light cones, and claim that some of those events are in the past but too far away to see yet. But someone moving slightly differently will claim different events are in the past. But everyone will agree on which events are in the past light-cone, which is also the only kind of "past" that has an observable, experimental effect.

(more...)

>> No.5340986

(contd)

2. You can't tell "when they arrive" at a distant point unless you have a universal notion of simultaneity/time. Your perception of simultaneity depends on how you're moving. (Further, your notion of "at the same distance" also depends on how you're moving.) All you can measure is stuff that happens to you / near you. So you could send a signal out, and have the remote transmitter send a response, and see when it gets back ... the book you're reading should have some discussion of experiments like this, thinking about them is actually the easiest way to get from "light speed is always constant" to "therefore there can't be an absolute notion of time".

3. You can have any *two* of: FTL travel, special relativity (w/o a preferred reference frame), and causality. If FTL travel is based on some absolute notion of time, then it doesn't break causality, but that means it wouldn't work the same in all reference frames. (Some science fiction novels have FTL drives that work like this.) If it does work the same in all reference frames, then you can go FTL from point A to point B, accelerate using normal rockets (thus changing your reference frame), then go FTL back to point A and get there before you left.

>> No.5340987

>>5340954
What book did you read?
I think it might have made you dumber.

>> No.5340994

>Hi, /sci/!
Hallo!

>1. Tying space-time to speed of light. SR assumes (amirite?) that before we observe the event, it doesn't happen.
the reason why time is not absolute is because physics is the same for all reverence frames, the things you are talking about are derived, not assumed. the only way to have the speed of light be constant for 2 people traveling at some speed relative to each other is to have time slow down for one of them.

>2. The book I read the last made a big point of an impossibility of calculating absolute velocity of a celestial body. Buuuuut don't we have speed of light as that absolute constant? Why couldn't we calculate our absolute velocity by sending radio signals to six different relays, positioned at the same distance from transmitter and recording time when these signals arrive?
no matter how fast you are going, you will measure the speed of light as the same, this will only allow you to determine your velocity relative to the 6 relays.

>3. Time dilation. The most WTF concept for me.
direct result of the speed of light being the same for everyone, see 1.

>4. Impossibility of traveling faster than light being based on it violating causation laws.
what is hapening for you at this moment is not the same as a car driving past you, when you look at something (even compensating for the time the light took to travel to you) and you see event A happening now, the guy in the car is seeing how it was before A happened, so if you then tell him what happened and he travels instantly to where A should happen, he will be there before it happened.


>Well, all of it boils down to "why the hell time is not an absolute value in SR?".
see 1.

>> No.5340995

Edgy teenager pseudo-philosophy goes to >>>/b/ >>>/r9k/ >>>/v/ >>>/reddit/ >>>/facebook/ >>>/9fag/

>> No.5340996

Not science. A 13 year old's first attempt at pseudo-philosophy belongs on reddit, not on /sci/. Here you have to be at least 18 and the topics have to be related to science and math.

>> No.5341004

>>5340995
>>5340996
lelz, so edgy

>> No.5341008

>>5341004
How old are you? I'd guess maybe 15. This shit isn't science.My dog can comprehend high schooler special relativity better than you.

>> No.5341016

>>5341008
im not even OP, what are you talking about?

>> No.5341426

>>5340954
I think you may be retarded, and not in the good autistic way where you have a super power.

>> No.5341648

All of your difficulties with relativity stem from not understanding the Lorentz transformation. The geometry of space time is not Euclidean, it is hyperbolic. Human spatial reasoning is based on the assumption of Euclidean geometry, hence, the geometry of space time can be difficult to grasp.

http://en.wikipedia.org/wiki/Hyperboloid_model
http://en.wikipedia.org/wiki/Lorentz_transformation

>> No.5341675

>>5341008
What lucky country are you in where they teach relativity in high school?

>> No.5341681

>>5341675
we did special relativity in a physics course i took in highschool in usa

it's based on highschool algebra

>> No.5341693

OP, read this: http://pdfcast.org/pdf/special-relativity-2

I love how nobody looks at the sticky.